As is well known, the use of lean premixed fuels in burners result in the production of a significantly lower amount of oxides of nitrogen (NO.sub.x) and many attempts have been made to provide lean premixed fuel burner capable of burning lean premixed fuels. The term lean fuel mixture or lean premixed fuel as used herein is intended to mean a fuel mixture having an equivalence ratio of less than unity (1), preferably less than 0.75, and the term equivalence ratio means actual fuel to air (oxidant) weight ratio divided by the stoichiometric fuel to air weight ratio. Obviously if another oxidant containing gases are used in place of air the equivalence ratio will change accordingly.
The use of lean fuel mixtures results in a lower flame temperature which may be reduced to a temperature at which the rate of NO.sub.x produced is small thereby lowering the NO.sub.x emissions while still generating a significant amount of heat.
One of the major problems encountered when burning a lean premixed fuel and oxidant mixture is flame instability and thus, which results in the flame being easily put out for example by being blown out, thereby rendering such systems defective.
WO95/09326 published Apr. 6, 1995, inventor Meijer et al. discloses one form of burner for burning the lean premixed fuel mixture by simply inducing more air at the base of the flame so that the fuel mixture leaving the nozzle or gun of the burner, may have a low equivalency ratio but this ratio is significantly increased by the air induced at the flame base to thereby maintain stability of the flame i.e. in the flame the equivalency ratio is or closely approaches 1. Meijer et al. provides stabilizer bridges across the nozzle outlets to help to maintain the stability of the individual flames which burn one directly above each stabilizer bridge.
It is also known to stabilize the periphery of a lean premixed fuel burner as described, for example, in the publication "Low NO.sub.x Production Through Lean Premixed Combustion" by Johnson et al. presented at the American Planning Research Committee on April 1994 and "Lean Burn Technology for Gas Appliances" by Johnson and Kostiuk, presented at the Canadian Conference of Applied Mechanics--May 28 to Jun. 1, 1995 which describe a burner having a peripheral ring stabilizer and the effective burning of lean fuel mixtures. In this system it has been found that the main flame extends from the nozzle outlet a very substantial distance in the order of (28 cm for a 3.2 cm diameter burner nozzle burning fuel with an equivalence ratio of 0.7) which makes the device difficult and, in fact, in many cases unsuitable as a replacement into conventional burner as the flame is too long.
It is also known to use in a conventional air fuel burner (as opposed to a lean fuel burner) to burn the premixed fuel within a ceramic cavity and to contain the burning fuel within the cavity by a screen mesh or the like extending across the side of the cavity remote from the where the fuel is injected. Such a system is shown in U.S. Pat. No. 5,326, 257 issued Jul. 5, 1994 to Taylor et al.
It is also known to apply a screen over a Bunsen burner or the like (i.e. for fuel mixtures with an equivalency ratio above 1) which breaks up the flame into a plurality of flames. These screens are not used as flame stabilizers but only function as stabilizers on the principal of the screen forming a heat sink at a temperature above the auto ignition temperature of the fuel mixture. This is dangerous as it may cause the flame to propagate backwards in the direction of fuel flow on the upstream side of the screen., i.e. burning back through the fuel system which is dangerous.
U.S. Pat. No. 5,447,427 issued Sep. 5, 1995 to Suzukli burns a low calorific gas in burner that receives the premixed fuel through a refractory baffle that is heated to a high temperature and sustains the burning of the low calorific gas in the recirculation zones on the downstream side of the refractory baffle.
U.S. Pat. No. 4,397,631 issued Aug. 9, 1983 to Fisher discloses a burner for premixed power gas wherein a burner plate of defined thickness and having a selected pattern of holes of selected size is used to prevent flashback and noise or screech while also improving flame stability.
U.S. Pat. No. 5,236,327 Issued Aug. 17, 1993 to Flanagan et al. discloses a burner for premixed lean gas mixture by reducing the time the combustion gases are in the flame zone by increasing the velocity of the fuel mixture and incorporates a flame stabilizer using a buff or blunt body in the premixed fuel passage to cause turbulence on its downstream side and tends to prevent the flame from "lifting off" due to the high gas velocities used. The flame length or height generated by this type of burner is very long.